dt-bindings: marvell,prestera: Add common compatible string

Add "marvell,prestera" as a compatible string so that drivers can be
written to account for any prestera variant without needing to
specialise to the more specific values.

Signed-off-by: Chris Packham <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Gregory CLEMENT <[e-mail]>

Documentation/devicetree/bindings/net/marvell,prestera.txt

···
 -------------------------------------
 
 Required properties:
-- compatible: one of the following
+- compatible: must be "marvell,prestera" and one of the following
 	"marvell,prestera-98dx3236",
 	"marvell,prestera-98dx3336",
 	"marvell,prestera-98dx4251",
···
 	ranges = <0 MBUS_ID(0x03, 0x00) 0 0x100000>;
 
 	packet-processor@0 {
-		compatible = "marvell,prestera-98dx3236";
+		compatible = "marvell,prestera-98dx3236", "marvell,prestera";
 		reg = <0 0x4000000>;
 		interrupts = <33>, <34>, <35>;
 		dfx = <&dfx>;